Fluorescence is the result of a three-stage process that occurs when certain molecules absorb energy. The three stages comprise: 1) excitation; 2) excited-state lifetime; and 3) fluorescence emission. During stage 1, excitation, a photon of a certain energy is absorbed by the fluorophore. The fluorophore is initially in its ground state (S0). Absorption of the photon causes that fluorophore to become excited. The energy of the absorbed photon is transferred to an electron. The electron is transferred to a higher energy state. The fluorophore exists in an excited electronic singlet state (S1), also called an excited state. The excited state of the fluorophore exists for a finite time, typically 10−8 to 10−9 seconds. During the excited state, the fluorophore changes in its translational, vibrational, and electronic energy states, and is subject to interactions with its molecular environment. The excited fluorophore releases energy and returns to the ground state, S0, by fluorescence emission. Other processes such as fluorescence energy transfer, intersystem crossing, and collisional quenching may also depopulate S1. The ratio of the number of fluorescence photons emitted, during the emission stage, to the number of photons absorbed, during the excitation stage, is termed the quantum yield. The quantum yield is a measure of the efficiency of fluorescence in competition with other processes such as fluorescence energy transfer, intersystem crossing, and collisional quenching.
During the third stage, fluorescence emission, a photon of energy hv (where h is Planck's constant and v is the frequency of the photon) is emitted, returning the fluorophore to its ground state S0. The energy of the emitted photon is lower than the energy of the photon absorbed during the excitation stage. The difference in energy can be attributed to dissipation through processes during the excited-state lifetime, such processes include fluorescence energy transfer, intersystem crossing, and collisional quenching. The difference in energy of the absorbed photon and the emitted photon is called the Stokes shift. The Stokes shift is fundamental to the sensitivity of fluorescence techniques because it allows emission photons to be detected against a low background, and at a different wavelength than the excitation photons.
Compounds that have fluorescent properties have numerous uses. Fluorescent molecules can be used in single molecule spectroscopy, liquid crystal displays, light emitting diodes, solar energy collectors, and laser active media. Fluorescent molecules whose spectra or quantum yields are sensitive to their environments are valuable as fluorescent dyes and in the study of heterogeneous media, organized media, and biological media.
Environment-sensitive fluorophores are a special class of chromophores that have spectroscopic behavior that is dependent on the physicochemical properties of the surrounding environment. Solvatochromic fluorophores display sensitivity to the polarity of the local environment. These molecules exhibit a low quantum yield in aqueous solution, but become highly fluorescent in nonpolar solvents or when bound to hydrophobic sites in proteins or membranes. Examples of solvatochromic fluorophores include 2-propionyl-6-dimethylaminonaphthalene (PRODAN) (Weber et al. Biochemistry 1979, 18, 3075-3078; Cohen et al. Science 2002, 296, 1700-1703), 4-dimethylamino phthalimide (4-DMAP) (Saroja et al. J. Fluoresc. 1998, 8, 405-410), and 4-amino-1,8-naphthalimide derivatives (Grabchev et al. J. Photochem. Photobiol., A 2003, 158, 37-43; Martin et al. J. Lumin. 1996, 68, 157-146). Although PRODAN and its derivatives are widely used, these probes have limitations resulting from the relatively intense fluorescence even in aqueous environments. Thus, there is a need for alternate compounds.
U.S. Patent Application Publication No. 2006/0234206 discloses a 6-dimethylaminonaphthalimide group as an environment-sensitive fluorophore for reporting protein/protein and peptide/protein interactions. The fluorophore is integrated as part of an amino acid termed Dap(6-DMN). U.S. Patent Application Publication No. 2006/0205760, to Hartsel et al., discloses a naphthalimide compound with a mono-substituted amino group at the 4 position. Hartsel does not disclose an amino acid motif as an imide substitutent.